Today there are a wide range of intravascular prostheses on the market for use in the treatment of aneurysms, stenoses, and other vascular irregularities. Balloon expandable and self-expanding stents are well known for restoring patency in a stenosed vessel, e.g., after an angioplasty procedure, and the use of coils and stents are known techniques for treating aneurysms.
Previously-known self-expanding stents generally are retained in a contracted delivery configuration using an outer sheath, then self-expand when the sheath is retracted. Such stents commonly have several drawbacks, for example, the stents may experience large length changes during expansion (referred to as “foreshortening” or “jumping”) and may shift within the vessel prior to engaging the vessel wall, resulting in improper placement. Another disadvantage is that after the stent is deployed it can experience longitudinal movement within the vessel (also referred to as “migration”), which can be attributed to repetitive longitudinal loading and unloading of the stent.
Additionally, repetitive loading and unloading of a stent have also been known to cause fatigue induced strut failure, which may contribute to restenosis and subsequent vessel narrowing and/or occlusion. Additionally, many self-expanding stents have relatively large delivery profiles because the configuration of their struts limits further compression of the stent. Accordingly, such stents may not be suitable for use in smaller vessels, such as cerebral vessels and coronary arteries.
For example, PCT Publication WO 00/62711 to Rivelli describes a stent comprising a helical mesh coil having a plurality of turns and including a lattice having a multiplicity of pores. The lattice is tapered along its length. In operation, the plurality of turns are wound into a reduced diameter helical shape, and then constrained within a delivery sheath. The delivery sheath is retracted to expose the distal portion of the stent and anchor the distal end of the stent. As the delivery sheath is further retracted, subsequent individual turns of the stent unwind to conform to the diameter of the vessel wall.
The stent described in the foregoing publication has several drawbacks. For example, due to friction between the turns and the sheath, the individual turns of the stent may “bunch up,” or overlap with one another, when the delivery sheath is retracted. In addition, once the sheath is fully retracted, the turns may shift within the vessel prior to engaging the vessel wall, resulting in improper placement of the stent. Moreover, because the distal portion of the stent may provide insufficient engagement with the vessel wall during subsequent retraction of the remainder of the sheath, ambiguity concerning accuracy of the stent placement may arise.
In another example, U.S. Pat. No. 5,603,722 to Phan et al. describes a stent formed of expandable strip-like segments. The strip-like segments are joined along side regions in a ladder-like fashion along offsetting side regions. A shortcoming of such a stent is that the junctions between adjacent segments are not provided with a means of addressing longitudinal loading. As a result, such a stent is susceptible to strut fracture.
In another example, U.S. Pat. No. 5,607,445 to Summers describes a balloon expandable stent. In one embodiment, the stent is constructed from a single wire that is configured so that each half of the wire is zig-zagged and curved to generally form a half-cylinder. The zig-zags of each half-cylinder are intermeshed so that they combine to form a cylindrical stent. The stent described in the foregoing publication has several drawbacks. The stent does not allow for longitudinal loading. As a result, applying a longitudinal load will cause the bends to move radially inward which will bias them into the vessel flow. Additionally, the stent design may be susceptible to fracture with repetitive loading and unloading.
In yet another example, U.S. Pat. No. 5,707,387 to Wijay describes a stent constructed from a plurality of bands, where each band is composed of a solid wire-like material formed into a closed, substantially rectangular shape. Each band is circumferentially offset from the adjacent band and adjacent bands are connected by one or more cross-tie members. This stent also has several drawbacks. The rectangular cell design does not allow for longitudinal loading because the cells are not flexible. Therefore, under a longitudinal load the apex will move out of plane and will be biased into the vessel (i.e., into the vessel flow). Secondly the stent may be susceptible to fracture with repetitive loading and unloading because of the rigid cells.
When utilizing stents in interventional procedures, it may be advantageous to deliver therapeutic agents to a vessel wall via the surface of the stent. Drug eluting stents have many advantages, such as controlled delivery of therapeutic agents over an extended period of time without the need for intervention, and reduced rates of restenosis after angioplasty procedures. Typically, the drug is disposed in the matrix of a bioabsorbable polymer coated on an exterior surface of the struts of the stents, and then gradually released into a vessel wall. The quantity of the therapeutic agent provided by the stent generally is limited by the surface area of the struts. Increasing the surface area of the struts may enhance drug delivery capability, but may compromise the overall delivery profile of the stent. There therefore exists a need for a prosthesis having a reduced delivery profile and enhanced drug delivery capabilities. This is especially beneficial if other attributes such as radial strength and flexibility are not compromised.
In view of the drawbacks of previously known devices, it would be desirable to provide apparatus and methods for an implantable vascular prosthesis comprising a plurality of helical portions joined together, wherein the prosthesis is configured to be used in a wide range of applications including maintaining patency in a vessel and delivering drugs to a vessel.
It also would be desirable to provide apparatus and methods for a vascular prosthesis that is flexible enough to conform to a natural shape of a vessel without substantially remodeling the vessel.
It further would be desirable to provide apparatus and methods for a vascular prosthesis having one or more optional radially expanding anchors that allow for additional control over the deployment of the vascular prosthesis at a desired location within a vessel.
It still further would be desirable to provide apparatus and methods for a vascular prosthesis that has a surface area that may be selected to facilitate in-vivo delivery of therapeutic agents without adversely impacting the mechanical properties (e.g., radial strength, flexibility, etc.) of the prosthesis.
It additionally would be desirable to provide apparatus and methods for a vascular prosthesis that has a strut configuration that allows for repetitive longitudinal loading and unloading of the prosthesis.
It further would be desirable to provide apparatus and methods for a vascular prosthesis that has a structure having the ability to absorb or distribute loads.
It yet further would be desirable to provide apparatus and methods for a vascular prosthesis that has a small delivery configuration to allow the prosthesis to be used in smaller vessels.